Piezoelectric bulk acoustic wave devices utilizing the mechanical resonance of a solid material have long been used in practice as oscillators and filters. As described in "Section II: Bulk Acoustic Wave Devices, Chapter 3" in "Handbook of Acoustic Wave Device Technology" (No. 150 Acoustic Wave Device Technology Committee of the Japan Science Foundation Ed., published in November 1991 by Ohm K.K.), the materials of piezoelectric substrates used in such devices are mainly quartz, piezoelectric ceramics such as PZT, LiTaO.sub.3 single crystal, and LiNbO.sub.3 single crystal. Quartz substrates are used as reference oscillators and reference filters which are stable with respect to temperature changes. Piezoelectric ceramic substrates are widely used as reference oscillators or voltage-controlled oscillators (VCO) in microcomputers, remote controllers, telephones, floppy disk drives, etc., second intermediate-frequency filters in communication equipment, and intermediate-frequency filters in AM and FM radio receivers. LiTaO.sub.3 single crystal substrates and LiNbO.sub.3 single crystal substrates are widely employed in clock signal-generating circuits of microprocessors, and used at a frequency of several megahertz to roughly 20 MHz in cameras, VCR's, telephones, and personal computers. These single crystal substrates are also used as oscillators of VCO circuits in frequency synchronizing circuits for digital signal processing, frequency modulating or detecting circuits of radio equipment, and motor control circuits, and additionally, used as filters in digital network timing extraction circuits.
Especially as oscillators for VCO, the prior art uses such oscillators as LC and CR, which suffer from several problems including a need for adjustment, lack of stability, and low C/N. Instead, oscillators using piezoelectric ceramic and LiTaO.sub.3 single crystal substrates are now used. FIG. 27 illustrates a prior art strip type piezoelectric bulk resonator for VCO circuits. This device has a X-cut LiTaO.sub.3 single crystal substrate 11 machined into a strip shape, and rectangular shaped electrodes 12 and 13 are attached to upper and lower surfaces thereof belonging to the X plane. The device is operated with a voltage applied to the electrodes 12 and 13 using taps extending therefrom. The device is combined with a varactor diode to construct an oscillator. FIG. 28 is a diagram showing a change of oscillation frequency when the control voltage across the varactor diode is changed. It seen from the diagram that for this device, the frequency variable width normalized at the oscillation frequency is about 0.5%. As described above, when a LiTaO.sub.3 single crystal substrate is used, an upper limit is imposed on the frequency variable width (the normalized frequency variable width about 0.5%). This is also true when substrates of piezoelectric ceramics such as PZT are used.
Since filters using piezoelectric ceramics, LiTaO.sub.3 single crystal and LiNbO.sub.3 single crystal substrates are characterized by compactness, adjustment-free, high selectivity, and frequency stability, they replace prior art LC filters, enabling to reduce the number of parts in peripheral circuits. For the filters using such substrates, however, the fractional bandwidth could not be increased beyond about 10%.
Further, on account of a large crystal grain size of piezoelectric ceramics, piezoelectric bulk acoustic wave devices using them experience greater resonance and insertion losses as the frequency becomes higher, leading to the problem that they can be used in practice only below 20 MHz. Also, while the upper limit of frequency is governed by the machining precision of a substrate thickness, the piezoelectric bulk acoustic wave devices using LiTaO.sub.3 single crystal substrates have an applicable limit below 100 MHz.
In order to use piezoelectric bulk acoustic wave devices at higher frequencies, a device was proposed of the structure wherein a piezoelectric film is formed on a substrate of YAG, spinel or sapphire. The device offers a high degree of freedom of material choice since the substrate need not be piezoelectric. The device, however, has the drawback that the effective electromechanical coupling constant is significantly lower than the effective electromechanical coupling constant inherent to the piezoelectric film since the substrate is thick as compared with the piezoelectric film. It was then proposed in 1980 to effect anisotropic etching of a Si substrate for local thinning to form a diaphragm, on which a piezoelectric ZnO film is formed. Since the thickness of silicon serving as the substrate can be significantly reduced, this process can realize a device capable of fundamental or overtone operation at a high frequency of at least 100 MHz. There is an additional advantage that even the design adapted to operate at a frequency of several gigahertz is relatively easy to fabricate due to the eliminated need for submicron patterning. However, in the devices constructed as above, since the frequency variable width and the fractional bandwidth depend on the characteristics of a piezoelectric film used, the limits on the frequency variable width and the fractional bandwidth remain unchanged as long as conventional piezoelectric materials are used.
As described above, oscillators for VCO using conventional piezoelectric bulk acoustic wave devices cannot be used in the application where a frequency variable width of at least about 0.5% is necessary, and conventional LC or CR oscillator devices are used in this application. On account of this, oscillators for VCO fail to eliminate adjustment, provide a good C/N, or reduce the number of parts in peripheral circuits.
For the global environmental protection, a regulation prohibiting the use of Pb will be enacted in the near future. Therefore, with respect to piezoelectric substrates for use in VCO, it is urgently needed to search a piezoelectric material substitute for Pb-containing piezoelectric ceramics.
The Japanese mobile satellite communications use a center frequency 2.4 GHz band with a bandwidth of 30 MHz. Accordingly, the manufacture of terminals for use in the communications requires oscillators featuring operation in this frequency band, size reduction, low phase noise, low spurious response, and a normalized frequency variable width of at least 1.25%.
Also recently, mobile communication systems as typified by cellular phone systems have grown to a big market. To comply with an increasing number of subscribers, it is under examination to introduce a system having a maximum bit rate of up to 2 Mbps as Phase 1 of the IMT2000 system. Studied for this system are the wide-band CDMA system in which the bandwidth is spread to 5 MHz/10 MHz/20 MHz and the TDMA system having incorporated frequency hopping. Since the former uses direct diffusion codes, it is believed that the available bandwidth is currently limited to 5 MHz to 10 MHz due to the radio interference among multiplex propagation paths, which makes it inevitable to develop a novel interference canceling technology. The latter needs a fast synthesizer for frequency hopping, which in turn, requires a VCO circuit operating in a center frequency 2 GHz band and featuring at least a frequency variable width of 20 MHz (normalized frequency variable width 1%). If a miniature oscillator having low phase noise and low spurious response so that it is applicable to this VCO circuit is realized, the technology will lead to an innovation in the communication system.
Further, the IMT2000 system aims as Phase 2 at a system with a bit rate of at least 2 Mbps capable of time-varying image transmission, and the frequency hoping TDMA system with a bandwidth of at least 20 MHz is regarded promising. Also needed in this case is a fast synthesizer, which in turn, requires a VCO circuit operating in a center frequency 2 GHz band and featuring a frequency variable width of 20 MHz to 60 MHz (normalized frequency variable width 1% to 3%) or greater. If a miniature oscillator having low phase noise and low spurious response so that it is applicable to this VCO circuit is realized, the technology will lead to an innovation in the communication system.